1. Field of the Invention
The present invention relates to a gas sensor for performing a correction of sensor output depending on concentration of a predetermined gas component in a measurement gas, a control device thereof and also relates to a method of measuring a NOx concentration.
2. Description of the Background Art
Conventionally, various measuring devices have been used for finding out concentration of a desired gas component in a measurement gas. A known device of measuring NOx concentration in a measurement gas such as a combustion gas, for example, is a gas sensor having a Pt-containing electrode and a Rh-containing electrode formed on an oxygen ion conductive solid electrolyte, such as zirconia (ZrO2) (see Japanese Patent Application Laid-Open No. 8-271476 and Japanese Patent Application Laid-Open No. 2004-37473, for example).
In a gas sensor according to the above, concentration of a predetermined gas component is measured by detecting a current which flows at an electrode (measuring electrode) of a sensor element used for such detecting depending on the concentration of the predetermined gas component or the like, as sensor output.
Some of the gas sensors according to the above have a protection film consisted of a porous body and formed on a measuring electrode used for detection of a predetermined gas component for protecting it (see Japanese Patent Application Laid-Open No. 10-38845, for example).
In the case an internal combustion is driven with a gas sensor disclosed in Japanese Patent Application Laid-Open No. 8-271476, Japanese Patent Application Laid-Open No. 2004-37473 and Japanese Patent Application Laid-Open No. 10-38845 mounted on an emission system of the internal combustion in an automobile engine or the like (in an actual use of the gas sensor), substances such as Na, Mg, Ca or the like included in exhaust gas sometimes dissolve to a water vapor, for example, thereafter to enter into a sensor element through a gas inlet provided for introducing air.
As disclosed in Japanese Patent Application Laid-Open No. 10-38845, for a gas sensor where a protection film of a porous body is formed on a measuring electrode for protecting it, substances such as Na, Mg, Ca or the like after entering into the sensor element sometimes cause a clogging of this protection film consisted of the porous body (hereinafter, substances such as Na, Mg, Ca or the like causing such clogging are also referred to as contaminants). Such a clogging gradually proceeds with the actual use of the gas sensor.
The clogging of the protection film of the measuring electrode caused by contaminants such as Na, Mg, Ca or the like is a main factor of sensitivity degradation of a gas sensor to a predetermined gas component which is a target of measurement (i.e. a sensitivity change of sensor output). Also, such sensitivity degradation of the gas sensor leads to deterioration of measurement accuracy.
When a predetermined gas component targeted for measurement is NOx, in order to adapt to the above sensitivity degradation, it is possible to correct a change of the sensor output caused by sensitivity degradation, by means of associating NOx concentration with the sensor output after sensitivity degradation. It is implemented by investigating the relation between NOx concentration and the sensor output with actual measurement for a previously prepared gas including a NOx component of given concentration by the gas sensor after sensitivity degradation.
On the other hand, when an internal combustion is driven with a gas sensor mounted on an emission system of the internal combustion in an automobile engine or the like (in the actual use of the gas sensor), it is not so easy to prepare a gas including NOx of a predetermined concentration in the emission system. Accordingly, in such situation, it is difficult to correct the sensor output by a method of measuring the sensor output with respect to a gas including NOx of given concentration after sensitivity degradation to associate them with each other.
From the above, the sensitivity degradation of the gas sensor mainly caused by the clogging of the protection film of the measuring electrode caused due to the actual use of the gas sensor is one of the factors to deteriorate measurement accuracy of the gas sensor. It is, therefore, necessary to suppress deterioration of measurement accuracy by the sensitivity degradation of the gas sensor. Further, it is necessary to correct the sensor output with stable precision to adapt to the sensitivity degradation of the gas sensor and to maintain high measurement accuracy, under a situation in which a predetermined gas component targeted for measurement cannot be prepared at a desired concentration.